I guess the question about the indie authors since the established author name itself acts as a brand.
In my opinion, on the Amazon, the cover. Yes, that is the key for the reader to enter your world of creation. Unfortunately, there are multiple creations with innumerable keys. So, first make sure the key is attractive enough for passers to grab it and try.
And ofcourse the content if you want to retain the readers to stay in your creative world.
As others have said, there is no magic strategy. Obviously, you need a professional cover, an attractive blurb and a good book. The cover must match the reader you are attracting, because it will be the basis of your ads. Even if your cover is beautiful, you may have to switch it out for a different cover, because your reader demographic is not what you expected it to be, or because your cover attracts too many clicks from the wrong readers. If you have a book that appeals to one Zon visitor in 10, you will obviously have a very different strategy from someone who has a book that appeals to one Zon visitor in 100, or one in 1000. If you have 10 grand to drop on promotion, you will obviously have a different strategy from someone who has a hundred bucks to their name.
The key is to know whom you are targeting. It is possible to have a high ranking and lose money. It is possible to have a low ranking and make money. Promo sites might give you great results, or they might just be a waste of your hard-earned cash. The same is true of AMS, fac*ebook, goo*gle ads, and everything else. There are no magic bullets, most self-published books sell fewer than 10 copies, most self-published authors will always have day jobs. The only way you will learn what works for you is by trial and error.